Each CPU has a "base" scheduling domain (struct sched_domain). These are
accessed via cpu_sched_domain(i) and this_sched_domain() macros. The domain
hierarchy is built from these base domains via the ->parent pointer. ->parent
MUST be NULL terminated, and domain structures should be per-CPU as they
are locklessly updated.

Each scheduling domain spans a number of CPUs (stored in the ->span field).
A domain's span MUST be a superset of it child's span, and a base domain
for CPU i MUST span at least i. The top domain for each CPU will generally
span all CPUs in the system although strictly it doesn't have to, but this
could lead to a case where some CPUs will never be given tasks to run unless
the CPUs allowed mask is explicitly set. A sched domain's span means "balance
process load among these CPUs".

Each scheduling domain must have one or more CPU groups (struct sched_group)
which are organised as a circular one way linked list from the ->groups
pointer. The union of cpumasks of these groups MUST be the same as the
domain's span. The intersection of cpumasks from any two of these groups
MUST be the empty set. The group pointed to by the ->groups pointer MUST
contain the CPU to which the domain belongs. Groups may be shared among
CPUs as they contain read only data after they have been set up.

Balancing within a sched domain occurs between groups. That is, each group
is treated as one entity. The load of a group is defined as the sum of the
load of each of its member CPUs, and only when the load of a group becomes
out of balance are tasks moved between groups.

In kernel/sched.c, rebalance_tick is run periodically on each CPU. This
function takes its CPU's base sched domain and checks to see if has reached
its rebalance interval. If so, then it will run load_balance on that domain.
rebalance_tick then checks the parent sched_domain (if it exists), and the
parent of the parent and so forth.

*** Implementing sched domains ***
The "base" domain will "span" the first level of the hierarchy. In the case
of SMT, you'll span all siblings of the physical CPU, with each group being
a single virtual CPU.

In SMP, the parent of the base domain will span all physical CPUs in the
node. Each group being a single physical CPU. Then with NUMA, the parent
of the SMP domain will span the entire machine, with each group having the
cpumask of a node. Or, you could do multi-level NUMA or Opteron, for example,
might have just one domain covering its one NUMA level.

The implementor should read comments in include/linux/sched.h:
struct sched_domain fields, SD_FLAG_*, SD_*_INIT to get an idea of
the specifics and what to tune.

Implementors should change the line
#undef SCHED_DOMAIN_DEBUG
to
#define SCHED_DOMAIN_DEBUG
in kernel/sched.c as this enables an error checking parse of the sched domains
which should catch most possible errors (described above). It also prints out
the domain structure in a visual format.
